1. Field of the Invention
The present invention relates to a solar cell module and a method of producing it. More specifically, the invention relates to a solar cell module which can be produced at a low cost, which shows a smaller energy conversion efficiency degradation even when the solar cell module is partly shaded and which is not damaged even when maintained in such state for a long time, and a method of producing such solar cell module. The present invention also relates to a solar cell module that can effectively exhibit a high energy conversion efficiency when installed on a curved surface, and a method of producing such solar cell module.
2. Related Background Art
Solar cells have been widely developed and are starting to be introduced for use in the home, as the environmentally acceptable clean power source. Ordinary electric appliances are usually driven with an alternating current of 100 V. On the other hand, the output of a solar cell is a direct current, and an inverter has to be used in order to obtain an alternating current. In such a case the inverter cannot operate efficiently unless the output of the solar cell is at least 100 V. Also when the electric power is stored in a secondary battery, the DC output can be directly used, but the secondary battery is usually used at 12 V to 24 V.
However, a unit cell of a solar cell can only provide an output voltage of 0.5 to 0.6 V, also a unit cell of an amorphous silicon solar cell of a relatively high output voltage can only provide an output voltage of 0.7 to 0.9 V alone, and the tandem cell of plural junctions stacked can only provide an output voltage of about 2 V at maximum. For this reason, the solar cell is usually used as a module in which plural unit cells are connected in series. Such formation of the module by series connection of the unit cells of the solar cell also provides an advantage of reducing the current in the module, thereby remarkably lowering the power loss resulting from electric resistance in the wiring part.
However, during the use of the module with the unit cells connected in series, there may generate a situation where the shade of a wood or a building falls on a certain unit cell of the unit cells constituting the module, whereby only the output of the certain unit cell extremely drops. Such a situation is called a "partial shade state". In such a partial shade state, the output voltage of the entire module dramatically drops, and in some cases the shaded unit cell merely functions as a load to cause heat generation or is damaged by a strong reverse bias voltage. In this regard, it is known to connect a small diode which is called a bypass diode to a solar cell element (unit cell) in parallel and in an opposite direction thereto, thereby reducing the influence of such a partially shaded state.
FIG. 4 shows the working principle of such a bypass diode, wherein unit solar cells 301 to 304 of the solar cell are connected in series and further connected to an external load 305. In a normal state under irradiation with sunlight 306, there is generated an output voltage equal to the sum of the output voltages of the unit cells 301 to 304 (in FIG. 4, reference character A indicates a terminal at a negative side, and B indicates a terminal at a positive side). However, when the shade of an object 307 falls, for example, on the unit cell 303, the unit cell 303 substantially functions as a load of an extremely high resistance, whereby the output current of the module is reduced and the output voltage is extremely lowered. In addition, the unit cell 303 receives, in the opposite direction, the sum of the output voltages of the unit cells 301, 302 and 304, whereby such reverse bias voltage causes abnormal heat generation in the unit cell 303 or damages the unit cell 303 by the reverse electric field.
On the other hand, in the case of providing a bypass diode 303' with the unit cell 303, the unit cell 303 is short circuited, whereby an originally intended current substantially flows in the entire circuit and the unit cell 303 can be protected from the reverse electric field. When the unit cell 303 functions in the normal state, the bypass diode 303' is reversely biased, whereby little leak current flows and the function of the module is not affected. The bypass diodes 301' to 304' are provided for the unit cells 301 to 304 of the solar cell, respectively, thereby causing damage by the reverse electric field in a unit cell unit.
However, such bypass diodes are required in the same number as that of the unit cells, and therefore the cost and complexity of the wiring step for this purpose is appreciable. For example, the Japanese Patent Application Laid-Open No. 3-24768 proposes that the bypass diode is integrally formed with the unit cell. In the prior art, as shown in FIGS. 9, 10, 11A and 11B, n-type diffusion regions are formed on both sides of a p-type substrate, and one of such diffusion regions is utilized as the bypass diode. In these drawings, reference character 20 indicates a p-type silicon substrate; 21 and 22, n-type diffusion layers; 23 and 24, electrodes; SC, a unit cell; BD, a bypass diode; 1, 2 and 3, unit cells; A and B, bypass diodes; C, a bypass diode; and 25, 26, 27 and 28, lead wires. Such a configuration, however, has drawbacks such as requiring a complex series connection step and also requiring one external diode per module. For this reason, the above-mentioned patent application also proposes a method of integrally forming an independent bypass diode in the substrate, as shown in FIGS. 12A, 12B, 13A and 13B. In these drawings, reference characters 61 and 101 indicate p-type silicon substrates; 62 and 102, n.sup.+ -type diffusion layers; 63 and 103, n-type diffusion regions; 64 and 104, p-type diffusion layers; 65 and 105, oxide films; 66 and 106, oxidation preventing films; 67 and 107, surface electrodes; 68 and 108, back surface electrodes; and 109, a junction short circuit portion. This configuration certainly simplifies the series connection step, but requires an additional semiconductor step for incorporating the diode into the substrate, so that this configuration cannot reduce the total manufacturing cost of the solar cell module.